Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. In the light of such trends, a great deal of research and study has been focused on secondary batteries which are capable of meeting various demands of consumers, and have high energy density and voltage.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbon material as an anode active material, and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode and addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into a carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and the cathode. Such a lithium secondary battery must be basically stable in an operating voltage range of the battery and must have an ability to transfer ions at a sufficiently rapid rate.
The non-aqueous electrolyte is incorporated into the battery at the final step of fabrication of the lithium secondary battery. Here, in order to reduce a period of time taken to fabricate the battery and optimize the battery performance, it is necessary to ensure rapid and complete wetting of the electrodes by the electrolyte.
As the non-aqueous electrolyte for the lithium secondary battery, aprotic organic solvents, such as ethylene carbonate (EC), diethyl carbonate (DEC) and 2-methyl tetrahydrofuran, are largely used. Such an electrolyte is a polar solvent having a polarity to an extent that can effectively dissolve and dissociate electrolyte salts and at the same time, is an aprotic solvent having no active hydrogen species. In addition, such an electrolyte often exhibits high viscosity and surface tension, due to extensive interactions occurring within the electrolyte. Therefore, the non-aqueous electrolyte for the lithium secondary battery exhibits a low affinity for electrode materials containing a binder such as polytetrafluoroethylene, polyvinylidene fluoride and the like, and therefore brings about a failure to achieve easy wetting of the electrode materials. Such a failure of easy wetting due to the low affinity, as will be illustrated hereinafter, is one of the primary causes which are responsible for deterioration of rate properties of the battery.
Further, with an increased cost of cobalt (Co) contained in lithium cobalt oxide (LiCoO2) that has been conventionally and largely used as a cathode active material for a secondary battery, many attempts have been made to lower costs of the cathode active materials by a use of various transition metal mixed oxides instead of LiCoO2 or by a combined use of them. However, such methods suffer from deterioration of high-rate discharge properties. Since the high-rate discharge properties significantly depend on the mobility of lithium ions (rate properties), it is possible to improve high-rate discharge properties and cling properties by enhancing the wettability of an electrolyte on the cathode active material to thereby improve rate properties.
In this connection, there are known a variety of techniques for improving the wettability of the electrolyte on electrodes. For example, Japanese Unexamined Patent Publication N). 2002-305023 and U.S. Pat. No. 6,245,465 disclose a method of improving the wettability of the electrolyte on the electrodes, by introducing into an electrolyte solvent, a fluoride (such as perfluorinated polyether) and sulfone substituted with various functional groups such as fluorinated alkyl.
Further, Japanese Unexamined Patent Publication Nos. 2000-082471 and 2003-096232 disclose an electrolyte with improved wettability by using an amphoteric surfactant (such as carboxylic acid salt type surfactant), or by using a copolymer of polyethylene oxide (PEO) and polypropylene oxide (PPO), which is substituted with a cross-linking agent having an oligoalkylene oxide structure.
Meanwhile, Korean Patent Application Publication Nos. 2001-066272 A1 and 1996-027036 A1 disclose polymer electrolytes which are prepared by mixing polyethylene oxide (PEO)- or polypropylene oxide (PPO)-based polymers with various lithium salts and metallocene, whereas Japanese Unexamined Patent Publication No. 1997-306501 discloses an organic electrolyte with an addition of an oleic amide surfactant which has an affinity for the organic electrolyte.
However, the secondary batteries of the aforesaid conventional prior arts were confirmed to suffer from the problems and disadvantages in that the overall operation properties of the fabricated battery are poor or at least a desired level of high-rate discharge properties is not exerted, due to high internal resistance.